In some air-conditioning apparatuses, like a multi-air-conditioning apparatus for a building, a heat source unit (outdoor unit) is installed outside a structure, and an indoor unit is installed in the indoor of the structure. A refrigerant that circulates through a refrigerant circuit of such an air-conditioning apparatus rejects heat to (removes heat from) air supplied to a heat exchanger of the indoor unit to thereby heat or cool the air. Then, the heated or cooled air is sent to an air-conditioned space to perform heating or cooling.
A building usually has a plurality of indoor spaces, and accordingly, such an air-conditioning apparatus also includes a plurality of indoor units. In the case of a large-scale building, the refrigerant pipe that connects the outdoor unit and each of the indoor units sometimes becomes as long as 100 m. When the length of the pipe connecting the outdoor unit and each of the indoor units is large, the amount of refrigerant charged into the refrigerant circuit increases accordingly.
Such indoor units of a multi-air-conditioning apparatus for a building are usually installed and used in an indoor space where humans exist (for example, an office space, a living room, or a shop). If, for some reason, a refrigerant leaks from an indoor unit installed in the indoor space, this may present a problem from the viewpoint of its effect on human body and safety, because some kinds of refrigerant have flammability and toxicity. Even if the refrigerant used is not hazardous to humans, it is conceivable that the refrigerant leak may cause oxygen concentration to decrease in the indoor space, which may exert an effect on human body.
The following method has been conceived to address this problem. That is, a secondary loop system is adopted for the air-conditioning apparatus, in which a refrigerant is used for the primary-side loop, and water or brine, which is not hazardous, is used for the secondary-side loop to provide air conditioning for the space where humans exist (see, for example, Patent Literature 1).
Aside from this problem, in the case of a multi-air-conditioning apparatus for a building, it is necessary to calculate the electric bill for each tenant using an indoor unit. Accordingly, indoor unit capacity is proportionally calculated in accordance with the usage capacity of each indoor unit as determined from, for example, the opening degree of an electronic expansion valve provided in association with each indoor unit. However, for the novel secondary loop air-conditioning system as described in Patent Literature 1, there is no method for calculating the load on each indoor unit, and it has been impossible to use a method conventionally adopted for a multi-air-conditioning apparatus for a building which uses a refrigerant.